0. L. Simmons — Development of Lungs of Spiders. 123 



but is not invaginated. He believes it is very probable that 

 the lungs were derived from the respiratory apparatus of some 

 Limulus-like, aquatic form. He thinks that tracheae are modi- 

 fied lungs. 



Laurie ('90), beginning with his stage K, describes the 

 changes in the abdominal segments of the Scorpion. At this 

 stage the pectinae and lung-book appendages are about equal 

 in size and structure. In stage L, the pectines have become 

 folded in a direction transverse to their axes. The other ap- 

 pendages are pushed in, forming little cavities (directed for- 

 ward) on the posterior sides of the appendages. In the stage 

 M the pectines are separated from the body wall at their distal 

 ends. The lung-book inpushings are deeper and the cavities 

 are divided up by lamellae. In the last stage described, (just 

 before hatching) the pectines and lung-books have much the 

 same structure as in the adult. 



Morin ('88)* states that the lungs of the dipneumonous 

 spiders arise in form of infoldings at the bases of the two 

 appendages of the second abdominal segment. At the an- 

 terior end of the sac on the dorsal side, is an infolding which 

 is the beginning of the lung leaves. The space between two 

 leaves connects directly with the body cavity. Two adjoining 

 leaves unite by the fusing of cells, as described by Locy. He 

 agrees with Locy as to the later stages. Morphologically the 

 lungs of Arachnida show great resemblance to the gills of 

 Limulus and similar forms. He emphasizes the position of 

 the infoldings on the posterior side of the appendage in both 

 cases. The lungs of Spielers are merely sunken gill-books of 

 Limulus. As the appendage sinks the stigma is left as an 

 opening between the posterior wall of the appendage and the 

 body wall. This author agrees with those who believe that 

 tracheae are modified lungs. 



It must not be forgotten that Elias Metschnikoif ('TO) de- 

 scribed some features of the lungs of the Scorpions but it is 

 not easy to understand either his text or his figures beyond the 

 fact that he states that the lungs develop behind the abdominal 

 limbs* 



As will be seen from the foregoing summary, the develop- 

 ment of the respiratory organs of the Arachnids has not been 

 followed throughout and the gaps in our knowledge are at just 

 the most critical points. To supply these deficiencies the inves- 

 tigations described below were undertaken. 



The work was done in the Biological Laboratory of Tufts 

 College. The eggs used were those of Agelena ncsvia and 

 Theridium tepidariorum. The eggs were killed in water 



* As summarized by Korschelt and Heider, '92, pp. 604-607. I cannot refer 

 to the original text. 



